TH 400 vs 350 vs 200 metric

[Alan Roehrich]

A low torque and HP car that turns a fair amount of RPM will see more gain from light weight transmission pieces than a high torque lower RPM car.

A car with less torque and HP that is turning a fair amount of RPM will often see fair amount of RPM drop.

The input drum spins at a percentage of the engine RPM, more specifically, the input drum is spinning at whatever the percentage of slip is in the converter at any given time.

On a normal 350 or 400, the 1-2 shift stops the direct drum, which has been spinning. Engine RPM drops, and so does the input drum RPM, so you accelerate the input drum again in 2nd gear, while holding the direct drum still. On the 2-3 shift, the direct drum instantly accelerates to the speed of the input drum as soon as the direct clutches engage fully. So 3rd gear shift you accelerate the direct and the input drum back up to whatever percentage of engine RPM the slip in the converter allows for.

Now, the further the RPM drop between gears, the more you have to accelerate those pieces. This soaks up some amount of torque, so, the less torque you have, the more there is to be gained with lightweight stuff.

Some of the higher HP cars have reported little or no gains from switching to a "trick" transmission such as a Pro Trans or a Kilgore, from a light 350, while others have reported 3-6 hundredths. The same applies to switching from a light 350 to a 200.

A fully lightened 400 can approach the same weight and efficiency as a 350, at a higher cost, with a greater margin of reliability, but at an increased cost, due to the cost of the gear set and the aluminum reaction carrier. The ET difference to a low RPM high torque car may not be enough to measure.

A car with a lot of torque may not even want or like a low gear lower than about 2.5:1, it may even slow it down, because of the extra time spent slipping the converter.

[Marvin Robinson]

Alan,

You are dead on, I'm running an F/SA 455 Olds .... it looks like the best bang for my buck would be either to "aluminize" my current TH400, or go with a properly built TH350. (I intend to run IHRA and some bracket events as well, so reliability is paramount - can't play the odds if the 200 metric is a weak link). The question of a lower 1st gear is still out there, the options are 2.48, 2.52, 2.75. I don't know whether the huge low end torque of the Olds needs a lower 1st gear or not when trying to harness it through a 9" tire. (I run Hoosier 30x9 radials). Right now, my power band does not go much beyond 5900, but a Bullet cam promises power up to 6400 or so. Don't wanna spin the heavy Olds stuff any higher with the components I'm forced to stick with in the rotating ***'y.

Hopefully I can gain enough enlightenment to get this combo right without burning thru cubic money I don't have....

[Alan Roehrich]

The 350 will cost less, and may be slightly faster. The low gear difference between the 350 (2.52) and the 400 (2.48) is negligible. I don't think I'd bother with a 2.75 low in your car. I'd be more inclined to buy the best converter possible. Of course, you'll be moving your crossmember and changing your driveshaft to accommodate a 350. The converter is the same, but your stall will go down when you put a converter that has been in front of a heavy 400 in front of a light 350. Maybe as much as 200-300 RPM.

I don't think you can make enough power or spin enough RPM to make reliability an issue with the bottom end of an Olds in Stock if it is well built. The stock F0 and F1 block is good to 600+ and 7000RPM (and that's 7000RPM while making good power, not 7000RPM going through the lights well after peak HP has come and gone). The stock cast crank will handle that as well, properly prepared.

[Marvin Robinson]

Alan, thanks so much for your expertise... it looks like I will be after a TH350, and have a converter done. I'll investigate the RPM potential next Spring after the Bullet cam goes in.

[TimKGillespie]

Marvin...you have a PM...

[Daran Summerton]

Forget the trans, it's worth 0 the converter can swing you 3-4 tenths!!!! I strongly advise using a BOP converter specialist. Stock eliminator converters will NOT work in a Olds 455.

Thats not true, you just need the right converter by the right company. The wrong converter is worth 3 10s the wrong way. Don't waste your time putting aluminum drums in a 400. Buy a good 350 or the right 200. A 200 will live fine behind a 455 if you have the turbo input converson, with a good pump and stator support.

Originally Posted by Daran Summerton

Forget the trans, it's worth 0 the converter can swing you 3-4 tenths!!!! I strongly advise using a BOP converter specialist. Stock eliminator converters will NOT work in a Olds 455.